I don't see it often, but I've seen it a few times in vector files: some of the shapes were left unclosed. The SE logo is not an example of this, but I modified it to illustrate my question:
You can see from the outline view that all the shapes are left un-closed. It has no effect on the rendered appearance, but I'm not sure why someone would be inclined to opt for this instead of closing the shapes.
The only reason that comes to mind would be to reduce the file size of the artwork. However, the amount of disk space saved for the example I provided comes out to be a whopping 236 bytes. Is there another reason for this sort of practice?